This invention relates to optical scanner devices, and more particularly to a mechanical resonant scanner having a mirror which moves to deflect light along a scanning path.
Mechanical resonant scanners are used in retinal display devices to scan an image onto the retina of an eye. In an exemplary configuration one scanner is used to provide horizontal deflection of a light beam, while another scanner is used to provide vertical deflection of the light beam. Together the two scanners deflect the light beam along a raster pattern. By modulating the light beam and implementing multiple colors, a color image is scanned in raster format onto the retina.
Scanning rate and physical deflection distance characterize the movement of the scanner's mirror. In the context of a retinal display the scanning rate and deflection distances are defined to meet the limits of the human eye. For the eye to continually perceive an ongoing image the light beam rescans the image, or a changing image, in periodic fashion. Analogous to refreshing a pixel on a display screen, the eye's retinal receptors must receive light from the scanning light beam periodically. The minimum refresh rate is a function of the light adaptive ability of the eye, the image luminance, and the length of time the retinal receptors perceive luminance after light impinges. To achieve television quality imaging the refresh rate is to be at least 50 to 60 times per second (i.e., .gtoreq.50 Hz to 60 Hz). Further, to perceive continuous movement within an image the refresh rate is to be at least 30 Hz.
With regard to the deflection distance, the mirror is deflected to define a raster pattern within the eye. System magnification and distance between the scanner and an eyepiece determine the desired deflection distance. To define a raster pattern in which millions of bits of information (e.g., light pixels) are communicated onto a small area (i.e., eye retina), the position of the mirror is controlled to a high degree of accuracy. In a conventional mechanical resonant scanner, the mirror position is controlled by a magnetic circuit. The magnetic circuit typically includes a pair of permanent magnets and a pair of electromagnets.
FIG. 1 shows a conventional scanner 10 having a mirror 12 and a spring plate 14. The mirror 12 and spring plate 14 are the only moving parts. The scanner 10 also includes a base plate 16 having a pair of stator posts 18, 20. Stator coils 22, 24 are wound in opposite directions about the respective stator posts 18, 20. The coil windings are connected in series or in parallel to a drive circuit. On opposite ends of the base plate 16, permanent magnets 26, 28 are mounted. The magnets are equidistant from the posts 18, 20. The spring plate 14 has enlarged opposite ends 30 that rest on a pole of a respective permanent magnet. The magnets are oriented to have the same pole in contact with each end of the spring plate 14. Thus, the opposite pole of each magnet 26, 28 is located adjacent to the base plate 16. The spring plate 14, magnets 26, 28 and the base plate 16 are tightly clamped together by respective caps 34, 36.
Magnetic circuits are formed in the scanner 10 so as to oscillate the mirror 12 about an axis of rotation 15. A first magnetic circuit extends from the top pole of the magnet 26 to the spring plate end 30, through an arm of the spring plate and mirror 12 across a gap to the stator pole 18, then through the base plate 16 back to the permanent magnet 26. A second magnetic circuit extends a similar path but through the stator post 20 instead of the stator post 18. A third magnetic circuit extends from the top pole of the magnet 28 to the opposite spring plate end 30, through an arm of the spring plate and mirror 12 across a gap to the stator pole 18, then through the base plate 16 back to the permanent magnet 28. A fourth magnetic circuit extends a similar path but through the stator post 20 instead of the stator post 18. A periodic drive signal is applied to the oppositely wound coils 22, 24 creating magnetic fields which cause the mirror 12 to oscillate back and forth about the axis of rotation 15. A pair of frequency adjustment screws 37, 38 can be adjusted so as to increase or decrease the tension in the spring plate 14. Variation of such tension increases or decreases the resonant frequency of the scanner 10.
The oscillation of the mirror back and forth along its path produces reactive forces in the scanner. Such reactive forces cause other components of the scanner to vibrate. The vibration of the scanner in turn causes whatever is attached to the scanner to vibrate. For a head-mounted virtual retinal display device, the display device vibrates on the viewer's head. Because the whole scanner is moving, this vibration typically causes the mirror position also to be in error.